1. Field of the Invention
This invention relates to an optical element consisting of a LiNbO.sub.3 or a LiTaO.sub.3 crystal doped with MgO or ZnO, and more particularly to an optical element having an improved optical damage resistant property.
2. Description of the Prior Art
Conventionally, LiNbO.sub.3 (Lithium Niobate, hereinafter referred to as LN) and LiTaO.sub.3 (Lithium Tantalate, hereinafter referred to as LT) are widely known as materials having an electro-optical effect or an acoustic-optical effect or as materials having a non-linear optical effect.
These LN and LT crystals are susceptible to a phenomenon in which irradiated light induces localized variations in refractive index, that is, optical damage. This optical damage becomes maximum when a linearly polarized beam enters while the direction of polarization of the polarized beam is coincident with the direction of the z axis of the crystal, whereby the incident beam becomes considerably fuzzy in the direction of the z axis. Even when the direction of the linearly polarized beams is not coincident with the direction of the z axis of the crystal, there are components of the linearly polarized beam in the direction of the z axis, and the crystal is prone to optical damage.
Conventionally, to increase the resistance to the optical damage, the doping of the LN or LT crystal with MgO or ZnO is widely practiced. For example, in the case of the LN crystal doped with MgO, it is said that the incident beam does not become fuzzy even when a laser beam having a wavelength of 515 nm and a power of several tens of milliwatts, which is output from an Argon laser, is incident on the LN crystal.
However, when the measurement of variations in refractive index is practically carried out with respect to a number of crystals, there are some crystals in which no optical damage occurs with an optical beam having a high output power of several tens of milliwatts, but the beam becomes fuzzy only with a relatively weak optical beam having an output power of several milliwatts or thereabouts, except for the crystals which are not susceptible to optical damage in the same manner as in the conventional crystal. This phenomenon, which the present inventor has observed, is completely reverse to the phenomenon which is conventionally known.
As with this result of observation, some reports suggest that the LN crystal doped with MgO is not necessarily resistant to optical damage. Specifically, it is reported in proceedings of the Applied Physics Association in spring, 1992, 30p-G-12 that the LN crystal doped with MgO and the LN crystal which is not doped with MgO have substantially the same optical damage resistant property.
The phenomenon of optical damage ascertained through the previously mentioned observation is observed only in the LN or LT crystal doped with MgO or ZnO, and such a phenomenon is not observed in non-doped crystals. Contrary to this, the phenomenon of optical damage which has been conventionally ascertained is naturally observed, even in non-doped crystals. The more the beam becomes fuzzy, the larger the optical power becomes. Therefore, the characteristics of the phenomena are different from each other. However, the phenomenon, in which a beam becomes fuzzy, is completely indistinguishable from that caused by conventional optical damage. Accordingly, this phenomenon is defined as optical damage throughout this specification, and it will be explained hereinbelow.